Methods of producing killed steel



United States Patent 3,414,042 METHODS OF PRODUCING KILLED STEEL Knut Franz Behrens, 86 Schlossstrasse, 422 Dinslaken,

Germany, and Walter Stein, 2 Heidestrasse, 4222 Friedrichsfeld, Germany Filed May 12, 1966, Ser. No. 549,615 4 Claims. (Cl. 164-57) The present invention is concerned with improvements in or relating to methods of producing killed steel.

When killed steels are made, the deoxidizing agents such as aluminum or silicon are mostly added to the liquid steel in the process of tapping the melting furnace into the teeming ladle. This mode of operation applies in particular to the production of killed mild steel of deep drawing grades, which are usually killed by means of aluminum only.

Especially in killed mild steel of deep drawing grades, but also in other steels killed by means of both aluminum and silicon together, or by any other deoxidizing agents, non-metallic inclusions are found time and again, located most usually underneath the surface of the steel ingot. The inclusions adversely affect the quality of the product and may eventually result in the necessity of chipping or scarfing the surface of the ingot or bloom. Such measures are expensive, increase the working time, and cause loss of iron.

Attempts have been made to remove the inclusions caused by deoxidization by increasing the holding time in the teeming ladle, but such step was found to be somewhat successful only with steels deoxidized by means of silicon. No improvement was obtained with this meas ure when aluminum was used to kill the steel. Such deoxidization products may largely be avoided by purifying the steel prior to adding the deoxidizing agents, using techniques such as the well-known vacuum degasifying. These measures are undoubtedly very successful, but again they seem to have little effect, especially on mild steel killed by means of aluminum only. In addition, they have the disadvantage of being rather expensive, thus tending to increase the price of the finished steel.

It is well known that rimming steel also presents inclusions but due to the boiling effect, especially during the initial stages of solidification in the mold, the skin layer of a rimming steel ingot is very clean. This has given rise to the idea of pouring the steel into the molds in a rimming state, and only then proceeding to deoxidize it in the mold by means of aluminum or any other agent. Thus, when pouring rimming steel into standard molds, the method widely used has been to introduce the deoxidizing agent, which has usually been aluminum, into the pouring jet toward the end of the pouring operation. Since aluminum dissolves well in steel and the pouring jet still provides for sufficient distribution and mixing in the ingot, there will be a relatively uniform distribution of aluminum throughout the entire ingot. Nevertheless, it happens frequently that the pouring jet, and consequently the deoxidizing agent, do not reach the bottom of the ingot, leaving the lower part of the ingot with little or no aluminum at all. Another disadvantage of this well-known technique is that the clean skin layer, which can only build up during the filling of the mold while aluminum is added, is very thin. Finally, the ingot pipe and porous locations in the ingot top have been found to be highly contaminated with deoxidization products, and it was established that such cavities (which are inevitable with killed steel) would not weld up dur-' ing rolling but result in laminations in the rolled material.

Another method is to allow the rimming steel first to boil in the mold for a few minutes to build up a sufficiently thick and clean skin layer, and then to add the ice deoxidizing agent. Although attempts have been made to push the deoxidizing agent down into the lower parts of the ingot, or to inject it in the form. of wire, a method is not as yet known of adding the aluminum that provides a sufiiciently good and uniform distribution of the deoxidizing agent throughout the entire ingot especially when making aluminum killed deep draw steel with pancake type grain. In fact, it has always been found that there is more aluminum at the top than at the bottom of the ingot. The bottom of the ingot is frequently insufficiently killed, causing the entire ingot to be rejected, or producing at least qualitative differences in the finished product, depending upon the location of the insufficiently-killed portion in the ingot.

The production of satisfactory deoxidization in the mold implies three requirements:

(1) The clean skin layer which builds up during the solidification of the rimming steel must be thick enough, i.e., the deoxidizing agent must not be added until a few minutes after the start of solidification.

(2) The deoxidizing agent must be uniformly distributed throughout the entire length of the ingot.

(3) The ingot pipes must be free from deoxidization products.

These three requirements can be met by a process in accordance with the present invention, the process including the steps of pouring the steel in a rimming state into a mold fitted with a feeder head until the level of the steel penetrates into the lower part of the feeder head; thereupon interrupting the pouring for at least the period necessary to allow the steel to boil up intensely and achieve the circulation common in the solidification of rimming steel; and thereafter filling up the feeder head of the mold with steel while at the same time the deoxidizing agent is added to the contents of the mold.

A particular preferred process includes the following specific steps:

(1) Rimming steel is poured into a mold with a feeder head of suitable design to prevent piping, the level of the steel bath rising just enough to penetrate into the lower part of the feeder head.

(2) The steel is allowed to properly boil up or rim in the mold, usually for about 1-5 minutes, and it may eventually be allowed another 1-5 minutes more.

(3) Then pouring is continued, and the feeder head is completely filled, the deoxidizing agent being added at the same time into the pouring jet, or injected into the metal in the mold.

The deoxidizing agent primarily used is aluminum, preferably in shot form, but other deoxidizing agents may also be used in this way, such as silicon, titanium, zirconium, etc., alone or in combination.

An example of a process in accordance with the invention will now be described, by way of example, with reference to the accompanying drawing, the three figures of which show successive stages in the filling of a mold.

Thus, a mold 10 of any suitable conventional form is provided at its upper end with side boards 11 of a suitable heat insulating material, comprising a feeder head that will prevent piping of the cast ingot. The mold shown in FIGURE 1 has already been filled with rimmged steel 12 from a ladle 13 until the steel bath has penetrated at least slightly into the lower part of the feeder head to the level 14; in this particular embodiment it is preferred that the bath penetrate to about /3 to /2 of the height of the side boards 11.

The mold shown in FIGURE 2 has been allowed to stand for an initial period of about 1-5 minutes necessary to allow the steel to boil intensely and achieve the circulation common in the solidification of rimming steel, and preferably has been allowed to stand for another 1-5 minutes, giving a preferred period of from 2-10 minutes,

during which the pouring has been interrupted. The pouring of the steel has now been resumed with the addition of aluminum deoxidiser in the form of shot to the steel stream 15, such addition being made by any conventional means and being indicated diagrammatically herein by the arrow 16.

In the mold shown in FIGURE 3 the momentum of the steel stream from the ladle and the movement of the steel by the rimming action has distributed the deoxidiser throughout the core of the ingot, the ingot now comprising a core 12a of killed steel and an outer skin 12b of rimmed steel. The top of the ingot has been covered with a layer 17 of an insulating or exothermic material.

In the particular embodiment illustrated the feeder head is inserted into the mold, but it can also be placed on top thereof. Alternative designs of feeder head will be apparent to those skilled in the art.

It has been found to be of particular importance that, prior to completely filling up the mold and adding the deoxidizing agent, the steel should have boiled intensely in the mold under the usual conditions of orientated circulation of the liquid steel in the mold. Otherwise, as experience has shown, good distribution of the deoxidizing agent throughout the entire length of the ingot and into the bottom of the ingot is not obtained. The thicker the rimming skin layer has to be, the longer the holding time must be.

In practice, a particularly efficient operation is achieved by using a method which consists of filling up the first mold to the lower part of the feeder head; completely filling up the first mold after having filled the second mold up to the lower part of the feeder head; then filling the second mold after having filled the third mold up to the lower part of the feeder head, and so on.

In more general terms the steel is poured into a mold designated IL until it is filled up to the lower part of the feeder head; the pouring into the mold n is then interrupted for the required period and during this period the preceding partly filled mold nl is completely filled and the succeeding mold n+1 is partly filled; the pouring into mold n is then resumed with the added oxidiser until the mold is full, and so on.

The advantages of processes in accordance with the invention are as follows:

(1) A sufficiently thick skin layer is obtained which is virtually free from deoxidization products. The surface condition of the corresponding product is thus very good.

(2) There are no labour and material costs for chipping or scarfing of killed steel products made by the usual process.

(3) The product is uniformly deoxidized throughout the entire length of the ingot and to a sufficient level of concentration to achieve complete and stabilized deoxidization.

(4) Since prior to killing a considerable part of the liquid steel had solidified, after killing the piping volume of the steel will be smaller; consequently, also the pipe will be smaller, the feeder volume required to obtain a pipefree material will be reduced, and the yield of steel will increase.

(5) The piping cavities are virtually free from deoxidization products and weld up well.

Any kind of deoxidizing agent may be used in this process, provided however that the steel boils in the mold prior to killing. The process is not confined to mild, low-carbon steel grades since it is known that steel grades having a higher carbon percentage can more readily be brought up to boiling by adding oxygen, or any other additive which favours boiling, into the pouring jet or mold, in one form or another. The term deoxidizing agent is used in the appendant claims, but will be understood to include a single agent or any suitable mixture of agents.

What is claimed is:

1. A process for making killed steel by adding at least one deoxidizing agent during pouring, including the steps of pouring the steel in a rimming state into a mold fitted with a feeder head until the level of the steel penetrates into the lower part of the feeder head; thereupon interrupting the pouring for at least the period necessary to allow the steel to boil up intensely and achieve the circulation common in the solidification of rimming steel; and thereafter filling up the feeder head of the mold with steel while at the same time the deoxidizing agent is added to the contents of the mold.

2. A process as claimed in claim 1, wherein the said period for boiling and partial solidification is from 2 to 10 minutes in length.

3. A process as claimed in claim 1, wherein the said at least one deoxidizing agent is added in shot form entrained in the poured steel stream.

4. A process as claimed in claim 1, and comprising the steps of pouring the steel into a mold 11 until the level of the steel penetrates into the lower part of the feeder head thereof, interrupting the pouring into the mold n for the said period and during the said period pouring the steel into the preceding mold n1, that previously has been filled until the level of the steel has penetrated into the lower part of the feeder head thereof, to fill the said mold nl, and also pouring the steel into the succeeding mold n+1 until the level of the steel penetrates into the lower part of the feeder head thereof.

References Cited UNITED STATES PATENTS 1,387,792 8/1921 McKenna 16456 X 2,108,254 3/1936 Devaney l6457 X 2,206,194 7/1940 Kinnear 164--133 2,389,516 11/1945 Kinnear 16496 2,819,503 1/1958 Boucek 164-56 OTHER REFERENCES Production of Aluminum-Killed Steel by Mold Additions of Aluminum, by K. F. Behrens and R. Hammer, Journal of Metals, September 1967, vol. 19, No. 9, pp. -64, p. 60 especially relied on.

I. SPENCER OVERHOLSER, Primary Examiner.

V. K. RISING, Assistant Examiner. 

1. A PROCESS FOR MAKING KILLED STEEL BY ADDING AT LEAST ONE DEOXIDIZING AGENT DURING POURING, INCLUDING THE STEPS OF POURING THE STEEL IN A RIMMING STATE INTO A MOLD FITTED WITH A FEEDER HEAD UNTIL THE LEVEL OF THE STEEL PENETRATES INTO THE LOWER PART OF THE FEEDER HEAD; THEREUPON INTERRUPTING THE POURING FOR AT LEAST THE PERIOD NECESSARY TO ALLOW THE STEEL TO BOIL UP INTENSELY AND ACHIEVE THE CIRCULATION COMMON IN THE SOLIDIFICATION OF RIMMING STEEL; AND THEREAFTER FILLING UP THE FEEDER HEAD OF THE MOLD WITH STEEL WHILE AT THE SAME TIME THE DEOXIDIZING AGENT IS ADDED TO THE CONTENTS OF THE MOLD. 